Whenever the baby is fussy and needing to be fed, changed or held, toddler siblings suddenly have an incredible urge to also be picked up, have a ride in the laundry basket or get help retrieving a car from behind the couch. It's especially urgent if you were also in the middle of that one important task you've been trying to get done all week.
Note: this 'ambush' rule doesn't apply when you have company over who could possibly help you out. No. In those instances, the kids tag-team their demands, crises etc. so they are spread out over the visit. I think they plan this in advance. No socializing for you!
(PS: "Bucket Hat" is a game my son invented where he brings me a bucket that I have to put on my head, he runs off and hides, and I come find him, all the while wearing the bucket. For a toddler, he has an incredibly long attention span for playing Bucket Hat.)